1. Field of the Invention
The present invention relates to a condenser microphone in which a microphone unit section can be attached to and detached from a microphone case and, more particularly, to a condenser microphone in which a microphone unit section can be arbitrarily replaced with, for example, a nondirectional microphone unit section or a unidirectional microphone unit section and an increase in noise due to the fact that a microphone unit section is made replaceable can be suppressed.
2. Related Background of the Invention
Since an impedance of a microphone unit is high, an impedance converter composed of a field effect transistor (FET) etc. is used in a condenser microphone. Further, in order to fulfill performance necessary as a microphone or perform required functions, an output circuit, a low cut circuit, etc., are incorporated in the microphone.
On the other hand, there is a condenser microphone in which a microphone unit section can be replaced depending on purposes. For example, there is a condenser microphone, in which it is possible to selectively replace and attach a nondirectional microphone unit section and a unidirectional microphone unit section. A unit replacement mechanism of a condenser microphone capable of replacing its microphone unit section is generally designed into a structure in which a unit case that incorporates a microphone unit is attached to a microphone case by a screw. A sound signal to be converted into an electric signal and output in the microphone unit is transmitted to the microphone case side via an electric contact, its impedance is converted by an impedance converter contained in the microphone case, and the converted sound signal is output to the outside after being subjected to predetermined process such as low cut process.
FIG. 4 and FIG. 5 show a conventional example of a condenser microphone capable of replacing its microphone unit section depending on purposes. In FIG. 4 and FIG. 5, symbol 1 denotes a microphone case and symbol 3 denotes a microphone unit section. The microphone case 1 has a female screw 19 on the inner circumferential surface at the front end (the left end in FIG. 4 and FIG. 5) portion and the microphone unit section 3 has a male screw 32 on the rear end portion, and it is structured such that the microphone unit section 3 can be attached to the microphone case 1 by screwing the male screw 32 into the female screw 19.
The microphone unit section 3 has a substantially cylindrical unit case 31, and a condenser microphone unit 30 is attached near the front end of the unit case 31. Since the condenser microphone unit 30 has a general unit configuration conventionally known, its detailed description is omitted. The front end of the unit case 31 is covered with a front cover 39 having a plurality of slit-like sound introduction holes. An inner side end plate 33 is engaged with and fixed to the rear end side of the unit case 31 and an outer side end plate 34 is engaged with and fixed to the further outside. A flanged contact 38 is inserted from inside into a hole formed at the center of the outer side end plate 34. A rod-like terminal 36 is inserted into a hole formed at the center of the inner side end plate 33 from outside toward inside and the terminal 36 is fixed to the inner side end plate 33 in a state of protruding toward the microphone unit 30. A compression spring 37 made of a conductor is interposed in the holes at the centers of the inner side end plate 33 and the outer side end plate 34, the contact 38 is pressed toward the outside of the outer side end plate 34 by a repulsion of the compression spring 37, and the rear end portion of the contact 38 which is formed into a semispherical shape protrudes from the outer side end plate 34. To the front end portion of the rod-like terminal 36, an output terminal plate 35 extending backward from the microphone unit 30 is screwed. Therefore, a sound signal converted by the microphone unit 30 is transmitted to the output terminal plate 35, the terminal 36, the compression spring 37, and the contact 38.
The microphone case 1 mainly comprises a relatively elongated cylindrical case 11. An end plate 16 is engaged with and fixed to the inner circumferential surface of the front end portion of the cylindrical case 11, located adjoining the female screw 19 and on the rear side thereof. A contact 20 is engaged with and fixed to the end plate 16 so as to penetrate its center in the direction of thickness. A circuit substrate 15 is attached to the inside of the cylindrical case 11 along its length direction. On the circuit substrate 15, an impedance converter circuit composed mainly of the FET, the low cut circuit, the output circuit, and the like are incorporated. The contact 20 is connected to an input terminal of the circuit substrate 15 via a proper conductive material. A connector case 12 is engaged with and fixed to a rear end portion of the cylindrical case 11 by a screw etc. A connector base 13 having insulation properties is fixed in the connector case 12. Three conductive pins 14 are buried in the connector base 13, constituting a connector having a standardized three-pin configuration. An output terminal and a ground terminal of the output circuit formed on the circuit substrate 15 are electrically connected with the corresponding pins, respectively.
FIG. 4 shows a state in which the microphone unit section 3 is attached to the microphone case 1 by screwing the male screw 32 of the unit case 31 into the female screw 19 of the cylindrical case 11. In this state, the contact 38 of the microphone unit section 3 comes into contact with the front end of the contact 20 on the microphone case 1 side and the contact 38 is pressed against the contact 20 by a compressive force of the compression spring 37, thereby the contact 38 and the contact 20 are electrically connected with each other. Therefore, a sound signal converted in the microphone unit 30 and output therefrom is input to the input terminal of the circuit substrate 15 via the output terminal plate 35, the terminal 36, the compression spring 37, the contact 38, and the contact 20. On the circuit substrate 15, a predetermined process such as impedance conversion, low cut, and amplification is performed and an output signal is transmitted to the three pins 14 as an output terminal. One of the three pins 14 is a ground pin connected to the ground. Into the connector case 12, a connector plug conforming to the standards is inserted, and a signal is input to an external circuit via the three pins 14 and a microphone cord.
According to the conventional condenser microphone described above, the microphone case 1 and the microphone unit section 3 are electrically connected with each other by screwing the male screw 32 into the female screw 19, therefore, the microphone case 1 and the microphone unit section 3 serve as a ground and function as a shielding member against an external electromagnetic wave.
However, the coupled portion of the microphone case 1 and the microphone unit section 3 is a path of an electromagnetic wave that enters from the outside and also a path of a signal of a sound signal on the grounding side, therefore, its structure allows a sound signal to be subject to the electromagnetic wave. In other words, the coupled portion by screw of the microphone case 1 and the microphone unit section 3 has an impedance for a high frequency signal, therefore, an electromagnetic wave is likely to enter into an internal circuit from the coupled portion and the electromagnetic wave that has entered into the internal circuit is detected by a semiconductor constituting the internal circuit and generates noises. In particular, when a signal current flows through the coupled portion by screw, the noises increase. Recently, a mobile phone prevails widely and in the situation in which a mobile phone is used frequently everywhere, it is more likely to occur that a mobile phone is used in the vicinity of a condenser microphone and an electromagnetic wave emitted from a mobile phone enters through the coupled portion and causes noises to be output from the condenser microphone.
As an invention described in a patent document relating to the condenser microphone according to the present invention, there is an invention as follows. In a condenser microphone in which a microphone capsule including a microphone unit is made replaceable to a grip part that incorporates an electric circuit for sound signal output, it is made possible to easily adjust a sensitivity when replacing a capsule by providing, for example, two capacitors for pads in the microphone capsule and making it possible to selectively connecting the capacitors for pads in parallel to the effective capacitance of the microphone unit using a selection switch (for example, refer to the patent document 1).
The object of the invention described in the patent document 1 is to make the microphone capsule including the microphone unit replaceable to the grip part and to make it possible to easily adjust the sensitivity when replacing the capsule. Therefore, as will be described later, its object is different from that of the invention of the present application, which object is to prevent noises due to the electromagnetic wave that tries to enter through the coupled portion of the microphone unit section and the microphone case in the condenser microphone in which the microphone unit section is made replaceable.
As an invention described in another patent document relating to the condenser microphone according to the present invention, there is an invention, that is, a unidirectional electret condenser microphone comprising a back electrode holder made of an insulating material and which engages and fixes a back electrode plate constituting an electrode on one side and a lead wire ring which is attached internally to the back electrode holder and electrically connects the back electrode holder with a printed circuit board, wherein an IC chip including a capacitor for removing high frequency noises, an inductor, and other circuit elements is contained in a back electrode chamber formed inside the back electrode holder and attached to the printed circuit board (for example, refer to the patent document 2).
The invention described in the patent document 2 comprises the inductor for removing high frequency noises, however, its object is different from that of the invention of the present application, which object is to prevent noises due to the electromagnetic wave that tries to enter through the coupled portion of the microphone unit section and the microphone case in the condenser microphone in which the microphone unit section is made replaceable.
As an invention described in still another patent document, there is a vibration pickup microphone using a ceramic bimorph element, in which even if a mobile phone is used near the microphone, a filter composed of an inductor and a capacitor is interposed between the ceramic bimorph element and a FET as an impedance converting circuit in order to avoid a digital noise from the mobile phone (for example, refer to the patent document 3).
The object of the invention described in the patent document 3 is to avoid a digital noise when a mobile phone is used in the vicinity of a microphone, therefore, its object is different from that of the invention of the present application, which object is to prevent noises due to the electromagnetic wave that tries to enter through the coupled portion of the microphone unit section and the microphone case in the condenser microphone in which the microphone unit section is made replaceable.
Further, a configuration is also known, in which an inductor as an impedance element is inserted in series with a transmission wave signal output line of a microphone at a position close to the microphone and the impedance element is used to prevent the intrusion of the radio transmission wave signal induced in the above transmission wave signal output line into the microphone (for example, refer to the patent document 4).
The invention described in the patent document 4 has disclosed that the impedance element connected in series with the transmission wave signal output line of the microphone are effective to avoid noises due to the electromagnetic wave, however, its object is different from that of the invention of the present application, which object is to prevent noises due to the electromagnetic wave that tries to enter through the coupled portion of the microphone unit section and the microphone case in the condenser microphone in which the microphone unit section is made replaceable.
[Patent document 1] Japanese Patent Application Laid-Open No. 2004-23705
[Patent document 2] Japanese Patent Application Laid-Open No. 2000-232700
[Patent document 3] Japanese Patent Application No. Hei 10-336794
[Patent document 4] Japanese Patent Application Laid-Open No. Hei 09-83463